1. Field of the Invention
The present invention generally relates to an X-ray CT (computerized tomographic) imaging method and an X-ray CT imaging system. More specifically, the present invention is directed to an X-ray CT imaging method and an imaging system capable of producing scano data from X-ray projection image data of a biological body acquired by helically scanning this biological body.
2. Description of the Prior Art
Generally speaking, to image a biological body under medical examination by utilizing a conventional X-ray CT system, a scano image (scanogram) of this biological body is first obtained by projecting X-ray beams to the overall portion of the biological body, with maintaining the X-ray source and detector at a fixed position. Based upon the scano image, positioning operation of tomographic images is carried out to determine a desired diagnostic portion of the biological body. Thereafter, while the X-ray source is rotated together with the X-ray detector around the diagnostic portion, this diagnostic portion of the biological body is scanned by way of the X-ray pulses to acquire X-ray projection image data, and then, the X-ray projection image data are processed to finally obtain X-ray CT image data. When such a scanogram is produced, while a couch on which the biological body is mounted is translated along a longitudinal direction of this biological body, the normal X-ray projection is performed without rotating the X-ray source around the biological body (namely, with fixing the X-ray source). Accordingly, the X-ray projection data are obtained and processed to form a scanogram 30 as represented in FIG. 1.
Subsequently, in the conventional X-ray CT imaging system, positioning of the CT images is performed with respect to this scanogram 30 at doted positions L.sub.1, L.sub.2, . . . L.sub.n ("n" being an integer) as shown In FIG. 1. Then, the couch on which the biological body under medical examination is once returned to the home position, and thereafter, is again translated to position the first CT imaging position "L.sub.1 " just under the X-ray source (not shown) and the X-ray detector (not shown either), which are rotated during the X-ray scanning operation. As a result, a series of X-ray projection data are acquired after the CT imaging operation at the final CT imaging position "L.sub.n " is accomplished.
As previously described, the conventional X-ray CT imaging system necessarily requires a lengthy examination time duration until the desirable CT images are obtained, since first the biological body is translated so as to obtain the scanogram 30, secondly this biological body is returned to the home position, and thirdly the biological body is again translated in order to obtain the X-ray CT image data. In other words, the biological body under medical examination such as a patient must endure such a lengthy and afflictive medical examination. Moreover, there are some risks that the relative position between the X-ray source and the patient differs from each other during the scanogram data acquisition and the CT image data acquisition. Even if, for instance, the first CT imaging position "L.sub.1 " is accurately determined on the scanogram 30, an actual CT imaging position on the patient may be positionally shifted.